This invention relates generally to condition sensors and more particularly to accelerometers having mechanically movable means responsive to acceleration for providing a corresponding electrical signal.
Condition sensors such as capacitive accelerometers and the like of various types as shown in U.S. Pat. Nos. 4,483,194 to Rudolph; 4,435,737 to Colton; Reissue 31,549 to Block and 3,240,073 to Pitzer are commonly used or proposed for use in aircraft and vehicular applications and the like where the sensors are likely to be subjected to shock, vibration and severe temperature changes but where it would be desirable for the sensors to be inexpensive and to display reliable and accurate performance characteristics over a long service life. However, many such sensors have limited performance capability or are manufactured at excessive cost. It would be desirable if such sensors could be provided with improved reliability in performance and could be made at reduced cost to find wider application.
In copending application, Ser. No. 07/790,956 assigned to the assignee of the present invention, compact, rugged and inexpensive accelerometer devices are disclosed comprising a stiff, rigid, electrically insulating ceramic substrate having a recess in the form of a groove formed in one substrate surface. Electrically conductive film means are deposited on that surface to define a capacitor detect plate inside the recess, a capacitor source plate connector pad outside the recess and circuit paths which are connected to the detect plate and source plate connector and to respective terminal pads along an edge of the substrate surface. The accelerometer device further includes a flat, electrically conductive plate or blade member of stiffly resilient metal which is formed with an attachment portion, a capacitor source plate portion and integral resilient beam means in a common plane. The attachment portion of the member is secured in electrically conductive relation to the source plate connector on the substrate with a thin layer of solder. Spacing between the upper surfaces of the detect plate and the source plate is determined by the depth of the recess. In one embodiment, glass frit including a bonding glass meltable at one temperature and glass rods of a small, precisely determined diameter which remain shape-retaining at the melting temperature of the bonding glass is deposited over two spaced portions of the source connector to provide a precise level of attachment of the source plate to the source plate connector.
Although devices made in accordance with the teachings of the above referenced patent application are very effective and perform well, the use of solder as a means of attachment involves a relatively time consuming reflow process and requires flux cleansing. In addition, there is a tendency for elements connected by solder to move slightly over time due to so-called solder creep thereby adversely affecting calibration of the device.
In copending application, Ser. No. 07/628,249 assigned to the assignee of the present invention, another accelerometer device is shown comprising an essentially flat electrically insulating substrate with a detect plate and a source plate connector disposed thereon and with a similar conductive blade member secured to the substrate in electrically conductive relation to the capacitor source plate connector. Spacing between the source plate portion and the detector plate is accomplished by using a shim between the attachment plate portion and the source plate connector in one embodiment and in another embodiment by reducing the thickness of the metal plate. However, use of a shim adds an additional part and process step which adds to the cost of the device as well. Further, when using a shim it is difficult to obtain close dimensional control from device to device. Use of a metal plate having a reduced thickness portion results in a more expensive blade member and one for which it is difficult to provide a properly balanced and mounted source plate portion.